Paper and board grades are produced in a paper machine by draining water out of a mixture of water and wood fibers prepared in a stock preparation plant and by using constantly moving fabrics in the different principal parts of the paper machine, which parts are typically called the wire part, the press section, and the dryer section. The fabrics in a paper machine are usually permeable members which form a closed loop, i.e., are endless, and which are made of a plastic and/or a metal material and/or which are felts consisting of natural and/or synthetic fibers. The fabric loops are rotated constantly by means of drive rolls or some other comparable equipment. During the draining of water from the mixture, the fabrics are contaminated by materials which come from the paper web and from the different process waters. In order that the fabrics and the elements in the paper machine, such as rolls, doctors, forming ribs, suction boxes, etc., may operate satisfactorily, they must be washed constantly or at least periodically by means of water jets, and the resultant wash water must be removed. The wash water from the fabrics is contaminated, but it can, however, be used as circulation water in the paper machine.
In present-day paper mills, an abundance of fresh water is needed for cooling and, after that, among other things, for the above washing requirements in the wire part and in the press section and for dilution in the stock preparation plant. After the wire part and the press section, these waters are passed mainly to mix with the fibrous circulation waters. Any excess amount of circulation water is disposed of as waste water. The net amount of fresh water that is needed for the washing jets in a paper machine is of an order of about 10 cubic meters per ton of paper produced. Thus, from a paper mill, an abundance of warm waste water is obtained, which must be cleaned, for example biologically, and, if necessary, cooled before the cleaning.
As known in the prior art, the wash jet waters in a paper machine are collected by means of various basins and troughs and passed into the circulation water system. Besides fresh water, circulation water of the paper machine is also employed as the jet water in the wire part and the press section. The circulation water is usually cleaned by means of filters having screens whose measure is about 150.mu. (corresponding to about 100 mesh). Such a screen measure, however, permits the passage of fine particles and dissolved material. A clear filtrate obtained from such a filtering device still contains finer particles and dissolved material. These impurities may cause blocking of jet nozzles and their structures and contamination of these devices and other equipment out of the disturbing materials in the water system in a paper machine, which results in negative effects in the quality and production of paper. The use of such water as an additional substitute for fresh water would risk damage to the efficient operation of the equipment and the production of paper. Therefore, for more demanding washing of the fabrics and parts of a paper machine, fresh water is used, which is often chemically cleaned prior to actual use in the paper machine.
Fresh water is generally cold, and it must be heated to a considerable extent, often to the operating temperature that is required in the papermaking process. The temperature of new, fresh cold water must be raised, for example, from about 7.degree. C. to about 50.degree. C., and usually it is treated chemically in order to remove humus materials and provide a proper color, in compliance with the quality requirements of the paper machine, and its use involves high processing costs. The high cost of cleaning of fresh water and waste water arises from the fact that an abundance of fresh water must be introduced into the process constantly. Fresh water which is used in the jets in a paper machine and which has been treated chemically also increases the concentrations of inorganic materials in the system.